The core rule book for the Labyrinth Lord game.
Labyrinth Lord is a retro clone game, using the OGL to create a ruleset that's compatible with the Moldvay/Cook B/X version of Dungeons and Dragons.
From the back of the book:
"Enter a world filled with labyrinths, magic, and monsters! You can take the role of a cleric, dwarf, elf, fighter, halfling, magic-user, or thief on your quest for glory, treasure, and adventure! This is a complete role playing game. All you need are a few sheets of paper and some dice. Welcome back to a simpler old-school gaming experience. The Labyrinth Lord awaits your arrival. Can you survive the dangers of the labyrinth?"